RFR (16): JFR: Separate metadata per JVM-feature

[Roman Kennke]

This is a fall-out from my Shenandoah upstreaming work in JDK11, where
I made a similar change in order to separate-out Shenandoah parts from
JFR build when Shenandoah is disabled.

Currently, all JFR metadata is collected in a single metadata.xml file.
>From those, the build machinery generates headers and some other things
from it. For JVM-feature-specific metadata, this leads to stuff
generated that doesn't exist when the feature is excluded from the
build. For example, when disabling Shenandoah, we still need to compile
empty methods in jfrPeriodic.cpp to satisfy the generated code. 

The fix is to extend the code-generator to accept multiple metadata-
*.xml files and concatenate the parsing. The version in JDK16 accepts a
--xml $FILENAME argument, and I'm extending this to --xml
$FILENAME1:$FILENAME2:.. etc, i.e. multiple filenames separated by
colon. It allows empty filenames, e.g. metadata.xml::: would be parsed
as a single file metadata.xml files. That makes the build machinery
much simpler.

I also did the relevant metadata-separation for Shenandoah because that
is what I care about myself. If you'd like other parts treated the
same, just let me know.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8250218
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8250218/webrev.00/

Testing: build with and without Shenandoah, run some tests, I'd await
submit-repo results before pushing.

Can I please get a review?

Thanks!
Roman